1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device, using a crystalline thin film and, more particularly, to a method of fabricating a planar thin-film transistor. Furthermore, the invention relates to a method of fabricating a liquid crystal display making use of such semiconductor devices.
2. Description of the Related Art
In recent years, technologies for fabricating field-effect thin-film transistors (TFTs) having excellent switching characteristics on substrates having poor thermal resistance, such as glass substrates and plastic substrates, have evolved, because with the development of techniques, amorphous silicon (a-Si) thin films and polysilicon (p-Si) thin films have been formed at lower temperatures.
At present, active matrix liquid crystal displays using a-Si thin films have become dominant in flat-display technology and almost form a vast field of the electronic industry.
In an active matrix liquid crystal display, millions of pixels are arranged in rows and columns, and TFTs are disposed at each pixel. Electric charge going into and out of each pixel electrode is controlled by the switching action of the TFTs.
TFTs using p-Si thin films have high field mobilities and operate at high speeds and so they permit fabrication of an integrated liquid crystal display incorporating peripheral driver circuits.
Accordingly, a liquid crystal display using p-Si thin film is recognized as a technique for accomplishing a next-generation, high performance intelligent display. It is considered that this technique will permit fabrication of an electronic system on glass (system-on-glass).
However, silicon films have their inherent problems. Amorphous silicon thin films and low-temperature p-Si thin films have high defect level densities due to dangling bonds and crystal grain boundaries. Therefore, when TFTs are manufactured, a hydrogenation step is necessary to conduct termination by hydrogen in the active layer.
Today, hydrogenation enjoys wide acceptance because it is effective in improving the electrical characteristics of TFTs such as mobilities, threshold voltages, off currents, and subthreshold swing factors. The hydrogenation is classified into two major methods: a method using thermal processing and a method using plasma processing.
In the former method using thermal processing, a substrate to be processed is heated in a hydrogen ambient at a temperature of 300-450xc2x0 C. for tens of minutes to several hours to thermally diffuse hydrogen into the thin film.
In this method, in order to shorten the hydrogenation time and to lower the equipment cost, the thermal processing is preferably performed at atmospheric pressure in a 100% H2 ambient. However, since hydrogen is very active (where certain content and environment temperature are exceeded, it explodes), the Industrial Safety Standard restricts the hydrogen content severely to 3-4% or less.
Accordingly, a method consisting of performing hydrogenation in an ambient of hydrogen diluted with an inert gas and a method consisting of carrying out hydrogenation at a low pressure of hundreds of torr have been proposed. Nevertheless, both methods suffer from low hydrogenation efficiency and offer only limited industrial practicability.
Another problem is that hydrogen diffuses itself into the active layer while kept in a molecular state and thus the probability that defect levels are terminated is not very high.
The latter method relies on plasma processing. Reactant gases such as H2, H2+O2, and NH3 are decomposed by a plasma discharge. The resulting hydrogen atoms are injected into the thin film.
In this case, the efficiency of hydrogenation is high but plasma damage and electrostatic discharge damage are induced. In addition, it is difficult to obtain optimum hydrogenation conditions.
It is an object of the present invention to provide a method of fabricating a semiconductor device, using a highly efficient hydrogenation method.
It is another object of the invention to provide TFTs having electrical characteristics which are improved by increasing the effects of hydrogenation.
In view of the foregoing problems, we have analyzed the prior art hydrogenation techniques and found that the following items play key roles in solving these problems.
(1) The diffusivity of hydrogen differs according to the material through which hydrogen diffuses. In the above-described method where hydrogen is diffused from outside, the amount of hydrogen reaching the active layer differs widely according to the material of the overlying layer. Therefore, it is important to perform hydrogen termination in an early stage of TFT fabrication process.
(2) Even if the active layer is terminated with hydrogen in an early stage of TFT fabrication process, hydrogen leaves the active layer at a certain probability when the layer is heated only to about 350xc2x0 C. Consequently, a hydrogen source for constantly replenishing hydrogen lost is necessary.
Accordingly, a method according to the invention is intended to provide a method of fabricating a semiconductor device on a substrate or base having an insulating surface, the method consisting of a first and a second steps. The first step consists of forming a region with a certain hydrogen content under an active layer that forms the semiconductor device. The second step consists of performing thermal processing to diffuse the hydrogen into the semiconductor device. The region formed by the first step is used as a hydrogen source. During the second step, the semiconductor device is hydrogenated, using this hydrogen source.
More specifically, the region with a higher hydrogen content than other regions has been previously formed under the active layer. Hydrogen termination is performed, using hydrogen supplied from inside, i.e., using the hydrogen-rich region as a hydrogen source. For this purpose, a heat treatment is made at a temperature of 300-450xc2x0 C. to thermally diffuse hydrogen out of the hydrogen source.
At this time, hydrogen ion implantation should be done without damaging a region which will become a channel later. For example, if the device is a normal staggered or normal planar TFT, the hydrogen ion implantation may be done after formation of a gate electrode. If the device is an inverse-staggered or inverted planar device, the implantation may be effected prior to the formation of the active layer.
For this reason, the hydrogen ion implantation is required to achieve considerable implantation depth. Consequently, use of ion doping is favorable. At this time, the dose is adjusted to be 1E15to 1E17/cm2.
Hydrogen ions introduced by ion doping collide with other atoms, so that energy is imparted to the hydrogen ions. Therefore, these hydrogen ions exist in atomic state. Hence, they can passivate the active layer efficiently.
Furthermore, the heat treatment for the hydrogen termination does not depend on the ambient because the hydrogen source is present under the active layer. This assures a stable hydrogenation efficiency. Moreover, this heat treatment can also perform the functions of other processing steps carried out at temperatures of 300-450xc2x0 C., because the heat treatment is independent of the processing environment.
The present invention also provides a method of fabricating a semiconductor device on a substrate or base having an insulating surface, the method comprising a first, a second, and a third processing steps. The first step consists of forming a region with a given hydrogen content under an active layer that forms the semiconductor device. The second step consists of forming an interlayer dielectric film from silicon nitride over the active layer. The third step consists of performing thermal processing to diffuse hydrogen into the semiconductor device. The region formed by the first step is used as a hydrogen source, and the semiconductor device is hydrogenated by the third step.
Since the diffusivity of hydrogen differs according to the material through which the hydrogen is diffused, if hydrogen is diffused from outside as in the prior art techniques, the amount of hydrogen reaching the active layer is affected greatly by the material of the overlying layer. Our experience shows that if the interlayer dielectric film is made of a silicon nitride film, hydrogen is blocked by this layer, making it difficult for the hydrogen to arrive at the active layer.
However, if hydrogen termination is performed before the silicon nitride film is formed over the active layer, there arises the problem that hydrogen leaves the active layer with a certain probability simply by heating the film to about 350xc2x0 C. in a later processing step.
In the present invention, a hydrogen ion source exists under the active layer and so the supplied ions are not blocked by the silicon nitride film or the like. Hence, a stable hydrogenation efficiency can be assured. Furthermore, the hydrogen ions from the hydrogen ion source can continually replenish hydrogen lost.
Since the hydrogen ions are diffused from inside the semiconductor device, if the silicon nitride film is used as an interlayer dielectric film, it also acts as a barrier film which blocks the hydrogen ions from escaping from the semiconductor device.
Accordingly, the interior of the device structure has a higher hydrogen content than TFTs fabricated by an ordinary hydrogenation method. Especially, where a silicon nitride film is used as a barrier film, the hydrogen ion content at the lower surface of the barrier film tends to be high.
While the configuration of the present invention has been described, the invention will be described in further detail, using its preferred embodiments.